Bonded joints are widely used in a variety of structural applications, and more specifically, primary composite structures are often bonded together in select aerospace applications. Generally, the strength of the bonded joint between composite structures must be known and certifiable if the bonded joint is used in a primary structure application. Conventional measurement of bond strength generally involves static and dynamic proof testing of entire structural assemblies, wherein several joints and structures are subjected to simulated loads and are monitored for strain levels, which are then correlated to strength values. The structure that is tested, however, is generally a test article and is not used in the final assembly of the aircraft. Further, smaller component testing of individual bonded joints is conducted, but the components are also test articles and are not a part of the operational vehicle structure. Moreover, the smaller components are most frequently destructively tested.
Non-destructive testing methods exist for composite structures, however, the non-destructive methods generally detect the presence of a defect such as a delamination or a foreign object within a laminate, not the strength thereof. Such methods may include, for example, ultrasonics, x-rays, and acoustics, among others commonly known in the art. Unfortunately, non-destructive testing methods to determine the bond strength of composite joints that are a part of a “fly-away” vehicle structure have not been developed to date in systems of the known art.
Methods have been developed to measure the strength of complete joints. However, the methods generally include the need to embed a foreign object within the composite materials in order to measure strength values. For example, U.S. Pat. No. 6,004,817 to Chamberlain et. al. discloses a method for measuring stress levels in polymeric compositions, including adhesively bonded joints. The stress levels are measured by detecting the magnetic characteristics of embedded microparticles that are dispersed uniformly throughout the composition.
Accordingly, there remains a need in the art for a non-destructive test method and system for bonded primary structures that determines the strength of a bonded joint. The system should be capable of non-destructively testing bonded joints that are a part of an operational structural system and should further be capable of testing a bonded joint under a variety of simulated load conditions.